Store Insurance
Most reputable tattoo shops are insured. The problem is, they’re usually insured against premises liability.
This means that they have insurance coverage if you fall and hit your head on their floor, but *NOT* if you’re unhappy with their work. In the past, the only insurer who would cover the latter was Lloyd’s of London, and their rates were apparently very high.
I personally recommend a tip for any work which you are pleased with, or any custom work where the artist spent time drawing up your illustration (since drawing time is usually not included in your price).
Nothing brightens up a day for the artist, or helps to build a friendly relationship with your artist more than a generous tip. If you’re very happy with the artist and you think you might get more work from them later, TIP!!
There have been heated discussions on rec.arts.bodyart in the past regarding the appropriateness of tipping a shop OWNER. If you feel that an owner does not deserve a tip on top of the price s/he charges you, then A) do not give a tip at all, or B) bring some sort of offering, be it food, flowers or whatever.
Many tattoo artists have told me that the BEST TIP is good word of mouth. If you are happy with your tattoo, show it off to your friends and tell them where you got it done!